Quantum mechanics is the study of matter and radiation at an atomic level. It is a mathematical tool for predicting the behaviors of microscopic particles1. Classical physics produced satisfactory results for systems that are much larger than atoms and moving with much slower than the speed of light. But it failed for the systems with small size (atomic size) or with speed near that of light. In the early 20th century scientists observed that results of certain experiments were not satisfactorily explained by classical physics. Particularly, it failed to account the observations in the following phenomena.